Cubic boron nitride sintered body and cutting tool

ABSTRACT

A cBN sintered body contains cBN particles whose proportion is 85-97% by volume, and a binding phase whose proportion is 3-15% by volume. The cBN sintered body contains Al whose ratio to the entirety of the cBN sintered body is 0.1-5% by mass, and Co whose mass ratio to the Al is 3 to 40, and includes Al3B6Co20.

TECHNICAL FIELD

The present invention relates to a cubic boron nitride sintered body anda cutting tool.

BACKGROUND ART

A cubic boron nitride sintered body (hereinafter usually referred to as“cBN sintered body”) has excellent wear resistance and is therefore usedas, for example, cutting tools. In particular, the cBN sintered body iswidely used to machine cast iron and sintered alloys that aredifficult-to-machine materials. For example, Patent Document 1 disclosesa cBN sintered body that is composed of a cubic boron nitride (cBN)phase and a binding phase containing W, Co, and Al, and containsB₆Co₂₁W₂ at a predetermined ratio as the binding phase. Patent Document2 discloses a cBN sintered body that contains 80% by weight or more ofcBN particles and Al whose content ratio exceeds 50% by weight. The Alis added together with Co as a binding phase.

RELATED ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Unexamined Patent Publication No.    2004-331456; and-   Patent Document 2: Japanese Patent Unexamined Patent Publication No.    2013-538937

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, the cBN sintered bodies respectively disclosed in the abovePatent Documents 1 and 2 have poor wear resistance because degranulationof cBN particles is apt to progress. Therefore, the wear resistancerequired, for example, when used as a cutting tool is far fromsufficient.

Means for Solving the Problems

A cBN sintered body according to the present embodiment contains cBNparticles whose proportion is 85-97% by volume, and a binding phasewhose proportion is 3-15% by volume. An Al content in the cBN sinteredbody is 0.1-5% by mass. A ratio of a Co content to the Al content is 3to 40 in terms of mass ratio (Co/Al). The cBN sintered body includesAl₃B₆Co₂₀.

Effects of the Invention

With the cBN sintered body according to the present embodiment, adhesionof the binding phase to the cBN particles is improved, deganulation ofthe cBN particles is inhibited, and excellent wear resistance isachieved. It is consequently possible to provide a cutting tool havingexcellent wear resistance and a longer service life by using the cBNsintered body as a cutting edge part of the cutting tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is X-ray diffraction data about an example of a cBN sintered bodyof the present embodiment; and

FIG. 2 is a schematic perspective view of an example of a cutting toolusing the cBN sintered body of the present embodiment as a cutting edgepart.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

A cubic boron nitride sintered body (cBN sintered body) contains cubicboron nitride particles (cBN particles) whose proportion is 85-97% byvolume, and a binding phase whose proportion is 3-15% by volume. An Alcontent in the cBN sintered body is 0.1-5% by mass. A ratio of a Cocontent to the Al content is 3 to 40 in terms of mass ratio (Co/Al). ThecBN sintered body includes Al₃B₆Co₂₀.

This improves the adhesion of the binding phase to the cBN particles andinhibits the degranulation of the cBN particles, thereby enhancing thewear resistance of the cBN sintered body of the present embodiment.Accordingly, a cutting tool having, at a cutting edge part thereof, thecBN sintered body of the present embodiment has excellent wearresistance and is therefore usable for a cutting process for a longtime. It is also possible to carry out wire electrical dischargemachining because the cBN sintered body has a low specific resistance.This makes it possible to manufacture an easy-to-machine cutting tool.

When the proportion of the cBN particles is less than 85% by volume, orthe proportion of the binding phase is larger than 15% by volume,hardness of the cBN sintered body is lowered and wear resistance islowered. When the proportion of the cBN particles is larger than 97% byvolume, or the proportion of the binding phase is less than 3% byvolume, the adhesion of the binding phase, namely, retention force ofthe cBN particles is lowered and the degranulation of the cBN particlesoccurs often, and consequently the wear resistance of the cBN sinteredbody is lowered. A volume ratio of each of the cBN particles and thebinding phase in the cBN sintered body according to the presentembodiment is obtainable in the following manner. An area ratio of eachof the cBN particles and the binding phase in a cross-sectionalphotograph of the cBN sintered body is measured, and obtainedtwo-dimensional area ratios are respectively taken as volume ratios ofthe cBN particles and the binding phase by regarding the two-dimensionalarea ratio as being the same as the three-dimensional volume ratio. Inthe measurement of the area ratios, the measurement is made at aplurality of locations, namely, three or more locations, and a meanvalue thereof is calculated. A preferable content ratio of the cBNparticles is 90-96% by volume.

When the Al content in the cBN sintered body is less than 0.1% by mass,no Al₃B₆Co₂₀ exists, and the adhesion of the binding phase is lowered,and the wear resistance of the sintered body is lowered. When the Alcontent is larger than 5% by mass, it becomes difficult to carry out thewire electrical discharge machining. When a ratio of a Co content to theAl content is lower than 3 in terms of mass ratio (Co/Al), no Al₃B₆Co₂₀exists and it becomes difficult to carry out the wire electricaldischarge machining. When the ratio of the Co content to the Al contentis higher than 40 in terms of mass ratio (Co/Al), the adhesion of thebinding phase is lowered, and the wear resistance of the cBN sinteredbody is deteriorated. A preferable range of the mass ratio (Co/Al) is 5to 30.

The mean particle size of the cBN particles is 0.2-10 μm, preferably 1-5μm, particularly preferably 1.5-2.5 μm in order to enhance the wearresistance and strength of the cBN sintered body in the presentembodiment. The hardness of the cBN sintered body is high and thedegranulation of the cBN particles can be inhibited as far as the meanparticle size falls in the above range. The measurement of the particlesize of the cBN particles in the present embodiment is made according tothe method of measuring a mean particle size of cemented carbide asdefined in CIS-019D-2005.

The binding phase exists while filling a clearance between the cBNparticles, and has a plurality of crystal phases and amorphous phasescoexisting therein. All ingredients other than the cBN particles are thebinding phase in the present embodiment.

In the present embodiment, as shown in X-ray diffraction data of Cu-Kαray in FIG. 1, among diffraction peaks of cBN (hereinafter usuallyreferred to simply as “cBN peaks,” this being also true for other peaks)in the cBN sintered body, peak strength of a peak belonging to (111)plane of the cBN (hereinafter usually referred to as “cBN (111) peak,”this being also true for other peaks) is the strongest. A Co (200) peakexists as a peak belonging to the binding phase.

According to the present embodiment, an Al₃B₆Co₂₀ peak exists at, forexample, 2θ=38.5°, 42.2°, 45.0°, 49.2°, and 51.3°. Of these peaks, thepeak at 2θ=38.5° overlaps with a peak of metal Al. Similarly, the peaksat 2θ=49.2° and 2θ=51.3° overlap with a peak of BCo. Therefore in thepresent embodiment, a determination whether Al₃B₆Co₂₀ exists or not ismade by the presence or absence of the peak at either one of 2θ=42.2°and 2θ=45.0°, which do not overlap with peaks of other crystal phases.Peak strength of the strongest peak of Al₃B₆Co₂₀ is the peak strength ateither one of 2θ=42.2° and 2θ=45.0°, which has greater strength than theother. Although the peaks whose diffraction angle (2θ) may varydepending on a slight difference of a solid solution state, residualstress, or the like, a determination can be made that the Al₃B₆Co₂₀ peakexists as long as the peak of each diffraction angle is detectable.

In the absence of Al₃B₆Co₂₀, the adhesion of the binding phase islowered and the cBN particles are apt to degranulate, and consequentlythe wear resistance of the cBN sintered body is lowered. Also in theabsence of Al₃B₆Co₂₀, it may be difficult to carry out the wireelectrical discharge machining of the cBN sintered body.

In the present embodiment, the peak strength of the strongest peak ofAl₃B₆Co₂₀ (at either one of 2θ=42.2° and 2θ=45.0°, whose peak strengthis higher than that of the other) with respect to the peak strength ofcBN (111) is preferably 0.02-0.25. When falling in this range, it ispossible to inhibit the degranulation of the cBN particles. A preferablerange of the peak strength of the strongest peak of Al₃B₆Co₂₀ withrespect to the peak strength of cBN (111) is 0.1-0.24.

Although a BCo peak is detectable in the vicinity of 2θ=42° only inSample No. 7 in FIG. 1, when no BCo peak exists or a strength ratio ofthe peak strength of the strongest peak of BCo to the peak strength ofcBN (111) is 0.05 or less, it is possible to inhibit the degranulationof the cBN particles.

Moreover, when the peak strength of the strongest peak of Al₃B₆Co₂₀ withrespect to the peak strength of Co (200) is 0.01-0.25, it is possible toinhibit the degranulation of the cBN particles and also carryout thewire electrical discharge machining. When the peak strength of Co (200)with respect to the peak strength of cBN (111) is 0.5-3, it is possibleto optimize the wear resistance and specific resistance of the cBNsintered body.

Besides the cBN peak, Co peak, and Al₃B₆Co₂₀ peak, a metal Al peak and aBCo peak may exist in the cBN sintered body. Although not being detectedin FIG. 1, an AlN peak may exist. In this case, peak strength of thestrongest peak of AlN with respect to the peak strength of cBN (111) ispreferably 0.05 or less. When falling in this range, it is possible toinhibit the degranulation of the cBN particles and also carry out thewire electrical discharge machining.

The cBN sintered body may further contain W. The W exists as metal W,WC, W₂B, B₆Co₂₁W₂, or the like. The metal W, WC, W₂B, B₆Co₂₁W₂, or thelike can be confirmed by detection of their respective peaks in theX-ray diffraction data. The cBN sintered body may still further containTi and other metal ingredients of periodic tables 4, 5, and 6.

A preferable range of the mean particle size of Al₃B₆Co₂₀ particles is0.02-0.5 μm in the present embodiment. This ensures that the Al₃B₆Co₂₀particles establish a strong bond to the cBN particles, therebyinhibiting the degranulation of the cBN particles. The mean particlesize of the Al₃B₆Co₂₀ particles in the present embodiment is found as adiameter of each particle which is obtainable by converting an area ofthe particle into a circle from a TEM observation.

An example of the cutting tool using the foregoing cBN sintered body asthe cutting edge part is described below with reference to FIG. 2. Acutting insert 1 (hereinafter referred to simply as “insert”) in FIG. 2has a cutting edge tip 5 composed of the cBN sintered body 6. Thecutting edge tip 5 is brazed to a front end of a tool body 10 with anback body 11 composed of cemented carbide containing WC and Cointerposed therebetween. The cutting edge tip 5 constitutes the cuttingedge part having a rake surface 2, a flank surface 3, and a cutting edge4 that is a ridgeline at an intersection of these two surfaces. The toolbody 10 is composed of cemented carbide or metal, such as high-speedsteel and alloy steel. Although only the cutting edge tip 5 is composedof the cBN sintered body 6 in the insert of FIG. 2, the presentembodiment is not limited thereto. Alternatively, the entirety of thecutting insert may be composed of the cBN sintered body 6.

A method for manufacturing the cBN sintered body is described below.

For example, 75-88% by mass of cBN raw material powder whose meanparticle size is 1.0-4.5 μm, 10-24.9% by mass of metal Co raw materialpowder whose mean particle size is 1.0-2.5 μm, 0.1-5% by mass of metalAl raw material powder whose mean particle size is 0.5-1.8 μm, and 0-20%by mass of WC raw material powder whose mean particle size is 0.3-1.5 μmare prepared. In the present embodiment, a ratio (cBN/Al) of the meanparticle size of the cBN raw material powder and the mean particle sizeof the Al raw material powder is set to 1.0-3.0, and a ratio (Co/Al) ofthe mean particle size of the Al raw material powder and the meanparticle size of the Co raw material powder is set to 0.7-1.6.

These prepared powders are pulverized and mixed together by a ball millfor 15-72 hours, followed by molding into a predetermined shape asneeded. The molding can be carried out using well-known molding means,such as press molding, injection molding, slip casting, and extrusionmolding.

Subsequently, this is loaded into an ultra-high pressure heatingapparatus together with a separately prepared back support body made ofcemented carbide, and is held at a predetermined temperature in a rangeof 1450-1700° C. at a pressure of 4-7 GPa for 10-60 minutes. Thetemperature is dropped at a rate of 3° C./sec or less. This ensures thatan abundance ratio of the cBN particles in the cBN sintered body can becontrolled in a predetermined range. This also allow the existence ofAl₃B₆Co₂₀.

In other words, when the mixing ratios of the raw material powders areout of the above-mentioned ranges, the abundance ratio of the cBNparticles in the cBN sintered body is out of the predetermined range, orno Al₃B₆Co₂₀ exists. When the ratio of the mean particle size of the cBNraw material powder and the mean particle size of the Al raw materialpowder, and the ratio of the mean particle size of the Al raw materialpowder and the mean particle size of the Co raw material powder are outof the above-mentioned ranges, the abundance ratio of the cBN particlesin the cBN sintered body is out of the predetermined range, or noAl₃B₆Co₂₀ exists. When the temperature drop rate is larger than 3°C./sec, there occurs no deposition of Al₃B₆Co₂₀.

The cutting edge part with a predetermined size is cut out from themanufactured cBN sintered body by the wire electrical dischargemachining. The cBN sintered body of the present embodiment has aspecific resistance that makes it possible to carry out the wireelectrical discharge machining. Subsequently, the cutting edge part socut out is brazed to a cutout step part formed at a corner part of afront end of a tool body made of cemented carbide. Thereafter, an uppersurface of the brazed insert is subjected to a grinding process, andaside surface of the cutting edge part is then subjected to a grindingprocess together with a protruded part of the cBN sintered body.Further, if desired, a cutting edge tip portion is subjected to a honingprocess, thereby manufacturing the cutting tool of the presentembodiment.

EXAMPLES

Firstly, cBN powder, metal Co powder, metal Al powder, and WC powder,which respectively had mean particle size presented in Table 1 weremixed into a composition presented in Table 1. The powder so obtainedwas mixed by a ball mill using alumina balls for 15 hours. The mixedpowder was then subjected to press molding at a pressure of 98 MPa. Themolded body and a back support body were stackedly set into theultra-high pressure heating apparatus. After raising temperature at 120°C./min and holding at a pressure of 5 GPa at 1450° C. for 15 minutes,the temperature was dropped at a temperature drop rate presented inTable 1, thereby obtaining a cBN sintered body.

Subsequently, an integrated object of the manufactured cBN sintered bodyand an back body was cut into a predetermined shape by the wireelectrical discharge machining. The results of machinability of the wireelectrical discharge machining were presented in Table 3, in which asample that had satisfactory machinability was indicated by symbol “O”,a sample that needed time to achieve the machining was indicated bysymbol “Δ”, and a sample that could not be machined was indicated bysymbol “x”. In the cutout step part formed at the cutting edge tipportion of the tool body made of the cemented carbide, a lower surfaceof the back body was brazed to a surface of the cutout step part of thetool body, and the cBN sintered body so cut out was made into a cuttingedge part. The cutting edge part of the cBN sintered body was subjectedto cutting edge processing (chamfer honing and round honing) by using adiamond wheel, thereby manufacturing a cutting insert of JIS⋅CNGA120408shape.

A content ratio of the cBN particles in the cBN sintered body of thecutting edge part in the obtained insert was calculated from a structureobservation of an arbitary cross section of the cBN sintered body byusing a scanning electron microscope (SEM). Additionally, a meanparticle size of the cBN particles was measured according to the methodof measuring a mean particle size of cemented carbide as defined inCIS-019D-2005. The results were presented in Table 1. In a structurephotograph, regions other than the cBN particles were regarded as thebinding phase.

Further, the composition of the cBN sintered body was checked by an ICPanalysis, and a content ratio of each of Co, Al, and W to the entiretyof the cBN sintered body was measured. The results were presented inTable 2.

Crystal phases in the cBN sintered body were specified by making anX-ray diffraction measurement from the rake surface and the flanksurface by using a D8 DISCOVER with GADDS Super Speed, manufactured byBruker AXS K.K, radiation source: CuK_(α), and collimator diameter: 0.8mmϕ. Table 3 presented peak strength of the strongest peak of peaksbelonging to each crystal phase when peak strength of cBN (111) was 1.XRD charts in FIG. 1 are XRD charts of Samples Nos. 7, 9, 10, and 11.

A cutting test was conducted using the obtained cutting inserts underthe following cutting conditions. The results were presented in Table 3.

Cutting method: external turning

Work material: FC250 sleeve material

Cutting speed: 700 m/min

Feed rate: 0.2 mm/rev

Depth of cut: 0.2 mm

Cutting state: under wet condition (with cutting oil)

Evaluation method: A cutting length before occurrence of wear orfracture was evaluated.

TABLE 1 Ratio of mean Mixed composition (Mean particle size: μ m,Additive amount: % by mass) particle sizes of cBN Co Al WC raw materialMean Mean Mean Mean powders Temperature Sample particle Additiveparticle Additive particle Additive particle Additive cBN/ Co/ drop rateNo. size amount size amount size amount size amount Al Al (° C./sec.) 13.2 63.6 1.3 34.1 1.5 2.3 — 2.1 0.9 2.0 2 3.2 71.2 1.3 27.0 1.5 1.8 —2.1 0.9 2.0 3 3.2 76.3 1.3 22.2 1.5 1.5 — 2.1 0.9 2.0 4 3.2 86.1 1.3 6.91.5 0.3 0.5 6.7 2.1 0.9 2.0 5 3.2 93.4 1.3 6.2 1.5 0.4 — 2.1 0.9 2.0 63.2 95.5 1.3 4.2 1.5 0.3 — 2.1 0.9 2.0 7 3.2 87.0 1.3 7.6 1.5 5.4 — 2.10.9 2.0 8 3.2 84.1 1.3 12.2 1.5 3.7 — 2.1 0.9 2.0 9 3.2 81.2 1.3 16.61.5 2.2 — 2.1 0.9 2.0 10 3.2 80.0 1.3 18.6 1.5 1.4 — 2.1 0.9 2.0 11 3.278.7 1.3 20.6 1.5 0.7 — 2.1 0.9 2.0 12 3.2 78.5 1.3 21.0 1.5 0.5 — 2.10.9 2.0 13 3.2 78.3 1.3 21.3 1.5 0.4 — 2.1 0.9 2.0 14 3.2 77.5 1.3 22.51.5 — — 2.1 0.9 2.0 15 3.2 68.1 1.3 4.4 1.5 0.12 0.5 27.4 2.1 0.9 2.0 163.2 80.0 2.5 18.6 1.5 1.4 — 2.1 1.7 2.0 17 3.2 76.2 1.3 18.8 1.5 5.0 —2.1 0.9 2.0 18 3.2 77.4 1.3 16.9 1.5 5.7 — 2.1 0.9 2.0 19 3.2 80.0 1.318.6 1.5 1.4 — 2.1 0.9 4.0 20 3.2 80.0 3.0 18.6 1.5 1.4 — 2.1 2.0 2.0 212.0 82.4 1.5 10.8 4.0 3.4 2.0 3.4 0.5 0.4 2.0 22 2.0 82.4 1.5 10.6 3.53.4 1.5 3.4 0.6 0.4 2.0 23 2.3 82.4 1.5 10.8 1.5 3.4 1.0 3.4 1.5 1.0 2.024 3.2 81.2 1.3 16.6 1.5 2.5 — 2.1 0.9 2.0 25 4.5 81.2 2.5 20.0 0.9 0.5— 5.0 2.8 2.0

TABLE 2 cBN sintered body Mean particle Mean particle size of Contentratio (% by volume) size of cBN Al₃B₆Co₂₀ Sample cBN Binding particlesparticles Content ratio(% by mass) No. particles phase (μ m) (μ m) Co AlW Co/Al 1 80 20 2.5 0.6 34.1 2.3 — 14.8 2 85 15 2.3 0.2 27.0 1.8 — 15.03 88 12 2.2 0.2 22.2 1.5 — 14.8 4 95 5 2.1 0.1 6.9 0.3 1.6 23.0 5 97 32.0 0.1 6.2 0.4 — 15.5 6 98 2 1.9 0.01 4.2 0.3 — 14.0 7 90 10 2.5 — 7.65.4 — 1.4 8 90 10 2.0 0.05 12.2 3.7 — 3.0 9 90 10 2.2 0.4 16.6 2.2 — 7.510 90 10 2.2 0.3 18.6 1.4 — 13.3 11 90 10 2.3 0.2 20.6 0.7 — 29.4 12 9010 2.4 0.04 21.0 0.5 — 40.0 13 90 10 2.4 — 21.3 0.4 — 45.0 14 90 10 2.0— 22.5 — — — 15 90 10 2.0 0.02 4.4 0.12 27.4 36.3 16 90 10 2.0 0.1 18.61.4 — 13.3 17 85 15 2.0 0.1 18.8 5.0 — 3.8 18 85 15 2.0 — 16.9 5.7 — 3.019 90 10 2.0 — 18.6 1.4 — 13.3 20 90 10 2.0 — 18.6 1.4 — 13.3 21 90 101.4 — 10.8 3.5 — 3.1 22 90 10 1.4 0.6 10.6 3.4 — 3.1 23 90 10 1.5 0.110.8 3.5 — 3.1 24 90 10 2.2 0.1 16.6 2.5 — 6.6 25 90 10 3.0 0.01 20.00.5 — 40.0

TABLE 3 Ratio of Wire Ratio of strength of strongest peak XRD ratios ofelectrical (Ratio to peak strength of cBN(111)) peak discharge Sample Costrengths of machining Cutting No. Al₃B₆Co₂₀ (200) AIN BCo Al₃B₆Co₂₀/Coperformance length (km) 1 0.25 1.2 — 0.28 0.21 ○ 5.9 2 0.17 1.2 — 0.070.14 ○ 9.1 3 0.15 1.2 — — 0.13 ○ 12.0 4 0.10 1.2 — — 0.08 ○ 14.7 5 0.061.2 — — 0.05 Δ 9.5 6 0.02 1.2 — — 0.02 Δ 4.5 7 — — — 0.25 — × — 8 0.050.5 — — 0.10 Δ 7.7 9 0.24 1.0 — — 0.24 ○ 13.8 10 0.20 1.2 — — 0.17 ○14.0 11 0.16 1.3 — — 0.12 ○ 13.7 12 0.04 1.5 0.10 — 0.03 ○ 7.6 13 — 1.50.22 — — ○ 4.3 14 — 2.0 — 0.17 — ○ 2.8 15 0.01 1.1 — — 0.01 ○ 9.3 160.18 1.5 — — 0.12 ○ 13.1 17 0.11 0.8 — — 0.14 Δ 9.7 18 — 0.6 — — — × —19 — 1.0 — 0.12 — Δ 6.8 20 — 1.0 — — — Δ 6.6 21 — 1.0 — — — Δ 6.2 220.11 0.4 — — 0.28 Δ 7.5 23 0.05 1.0 — — 0.05 ○ 9.0 24 0.27 1.1 — — 0.25○ 7.8 25 0.01 3.2 — — 0.003 ○ 7.2

The results presented in Tables 1 to 3 show the following. In Sample No.1 in which the content of the cBN particles was less than 85% by volumeand the content ratio of the binding phase was larger than 15% byvolume, the wear resistance was lowered, and the cutting length wasshort. In Sample No. 6 in which the content of the cBN particles waslarger than 97% by volume and the content ratio of the binding phase wasless than 3% by volume, the degranulation of the cBN particles occurredoften, the wear resistance was lowered, and the cutting length wasshort. In Sample No. 7 in which the ratio of the content of Co and thecontent of Al (Co/Al) was smaller than 3, and in Sample No. 18 in whichthe content of Al was larger than 5% by mass, no Al₃B₆Co₂₀ existed, andthe wire electrical discharge machining could not be carried out. InSample No. 13 in which the ratio of the content of Co and the content ofAl (Co/Al) was larger than 40, and in Sample No. 14 in which the contentof Al was less than 0.1% by mass, no Al₃B₆Co₂₀ existed, and the cuttinglength was short. In Samples Nos. 19-21, no Al₃B₆Co₂₀ existed, the wireelectrical discharge machining performance was poor, and the cuttinglength was short.

In contrast, in Samples Nos. 2-5, 8-12, 15-17, and 22-25 in which 85-97%by volume of the cBN particles and 3-15% by volume of the binding phasewere contained, the Al content was 0.1-5% by mass, the ratio of the Cocontent to the Al content (Co/Al) was 3-40, and Al₃B₆Co₂₀ existed, thewire electrical discharge machining could be carried out, wearresistance was excellent, and the cutting length was long.

In Samples Nos. 2-5, 8-12, 15-17, and 22-25, particularly each ofSamples Nos. 2-5, 8-12, 16, 17, 22, and 23 in which the peak strength ofthe strongest peak of Al₃B₆Co₂₀ with respect to the peak strength of cBN(111) was 0.02-0.25 in the X-ray diffraction measurement, the wireelectrical discharge machining could be carried out and the cuttinglength was long. In Samples Nos. 2-5, 8-12, 15-17, 23, and 24 in whichthe peak strength of Co (200) with respect to the peak strength of cBN(111) was 0.5-0.3, the wire electrical discharge machining could becarried out, and the cutting length was long.

Further, in Samples Nos. 2-5, 8-12, 15-17, 23, and 24 in which thestrongest peak of Al₃B₆Co₂₀ with respect to the peak strength of Co(200) was 0.01-0.25, the wire electrical discharge machining could becarried out, and the cutting length was long.

In Samples Nos. 2-5, 8-11, 15-17, and 22-25 in which a peak of AlN didnot exist or existed when a strength ratio of the peak strength of thestrongest peak of AlN to the peak strength of cBN (111) was 0.05 orless, and in Samples Nos. 3-5, 8-12, 15-17, and 22-25 in which a peak ofBCo did not exist or existed when a strength ratio of the peak strengthof the strongest peak of BCo to the peak strength of cBN (111) was 0.05or less, the wire electrical discharge machining could be carried out,and the cutting length was long.

In Samples Nos. 2-5, 8-12, 15-17, 23, and 24 in which the mean particlesize of the cBN particles was 1.5-2.5 μm and the mean particle size ofthe Al₃B₆Co₂₀ particles was 0.02-0.5 μm, the wire electrical dischargemachining could be carried out, the wear resistance was excellent, andthe cutting length was long.

DESCRIPTION OF THE REFERENCE NUMERAL

-   1: insert (cutting insert)-   2: rake surface-   3: flank surface-   4: cutting edge-   5: cutting edge part-   6: cBN sintered body

The invention claimed is:
 1. A cBN sintered body, comprising: cBNparticles whose proportion is 85-97% by volume; and a binding phasewhose proportion is 3-15% by volume, wherein an Al content in the cBNsintered body is 0.1-5% by mass, wherein a ratio of a Co content to theAl content (Co/Al) is 3 to 40 in terms of mass ratio, and wherein thecBN sintered body includes Al₃B₆Co₂₀.
 2. The cBN sintered body accordingto claim 1, wherein a ratio of peak strength of a strongest peak of theAl₃B₆Co₂₀ to peak strength of cBN (111) is 0.02-0.25 in an X-raydiffraction measurement.
 3. The cBN sintered body according to claim 1,wherein a ratio of peak strength of a strongest peak of the Al₃B₆Co₂₀ topeak strength of Co (200) is 0.01-0.25 in an X-ray diffractionmeasurement.
 4. The cBN sintered body according to claim 1, wherein aratio of peak strength of the Co (200) to peak strength of cBN (111) is0.5-3 in an X-ray diffraction measurement.
 5. The cBN sintered bodyaccording to claim 1, wherein no peak of AlN exists, or a ratio of peakstrength of a strongest peak of AlN to peak strength of cBN (111) is0.05 or less in an X-ray diffraction measurement.
 6. The cBN sinteredbody according to claim 1, wherein no peak of BCo exists, or a ratio ofpeak strength of a strongest peak of BCo to peak strength of cBN (111)is 0.05 or less in an X-ray diffraction measurement.
 7. The cBN sinteredbody according to claim 1, wherein a mean particle size of the cBNparticles is 1.5-2.5 μm and a mean particle size of Al₃B₆Co₂₀ particlesis 0.02-0.5 μm.
 8. A cutting tool, comprising a cutting edge partcomposed of the cBN sintered body according to claim 1.